Sakura could be a great choice. She has strong medical ninjutsu skills and a no - nonsense attitude. She could help the six - tails jinchuriki when they get injured during battles against the power of the six - tails or external enemies. Also, her growth as a character could be further explored as she tries to understand and support the jinchuriki.
One idea could be the six - tails jinchuriki's struggle to control the power. For example, they constantly have to battle the inner urge of the six - tails to go on a rampage. Another plot could involve the jinchuriki being hunted by an evil organization that wants to extract the six - tails' power for their own malicious use. They would then have to fight and outwit this organization while also dealing with the social ostracism that often comes with being a jinchuriki.
Often, other Naruto characters like Naruto Uzumaki or Kakashi Hatake might be included in the two - tails jinchuriki fanfiction. Their interactions with the two - tails jinchuriki can create new and exciting plotlines, whether it's them helping the jinchuriki control their power or going on adventures together.
Yugito Nii is a very popular character as she is the actual 2 - Tails Jinchuriki in the Naruto series. In fanfiction, she often takes center stage, whether it's in stories about her training or her battles against powerful foes.
One interesting story could be about the two - tails jinchuriki's journey to control the beast's power. Maybe they face a powerful enemy and have to learn to use the two - tails' abilities in a new way to defeat them.
One possible plot idea could be the Jinchuriki's struggle to control the power of the Five Tails Gobi. Maybe they face challenges from other ninja clans who want to use the power for their own gain. Another idea is that the Jinchuriki has to go on a journey to find a way to fully merge with the Five Tails without losing their own identity. And perhaps they form an unexpected alliance with a group of outcasts in the ninja world to protect the Five Tails from a greater threat.
One idea could be exploring the relationship between the 2 - Tails Jinchuriki and the other Jinchuriki in the Naruto world. Maybe they form an alliance to protect themselves from those who would use their power for evil.
The relationship could be a complex one. At the start, the Jinchuriki may not even be aware of the Five Tails Gobi's presence, but as they start to notice strange things happening within themselves, they begin to explore the connection. The tailed beast could be initially hostile, seeing the Jinchuriki as a weak host. However, as the Jinchuriki faces various challenges and grows stronger, the Five Tails Gobi starts to see potential in them. This could lead to a transformation in their relationship, from one of enmity to one of begrudging respect, and eventually to a deep and symbiotic bond where they rely on each other for survival and the protection of the ninja world.
Kabuto could also be a good antagonist. After he gained all those powers and knowledge, he might target Naruto's six - tails form. He could use his medical ninjutsu to try and manipulate Naruto's transformation or even try to extract the power of the six - tails for his own use. Since he has the ability to mimic other ninja's techniques, he could be a formidable foe against Naruto in six - tails form.
Well, a cool plot for Naruto six - tails form fanfiction might be about time travel. Naruto accidentally travels back in time while in six - tails form. He then has to figure out how to hide his form from his younger self and the people in the past. At the same time, he could use his knowledge from the future and the power of the six - tails to prevent some great disasters from happening in the past, which would change the future in unexpected ways.